1. Field of the Invention
The present invention relates to a power supply circuit and an electronic device supplied with power from the power supply circuit, and particularly relates to a power supply circuit provided with a backup system and an electronic device supplied with a power from the power supply circuit.
2. Description of the Related Art
In an electronic device (electronic equipment), such as a digital equipment, when the power is turned off (cutoff) and a voltage of a power supply and a current become unstable or when a power cannot be normally supplied, for example, due to an instant voltage down (temporary blackout), etc., the problems described below may occur.
(1) When the power supply voltage becomes low and an operation voltage of electronic device components becomes insufficient, the electronic device may runaway, for example, because values of a program and data read from a RAM in the electronic device become abnormal or an operation of a CPU in the electronic device becomes unstable, etc. Also, abnormal information may be stored in a recording medium, such as a flash memory and a disk.
(2) The flash memory, etc. is, for example, an EEPROM, so that the power supply has to be kept for a while to execute writing by receiving an instruction from the CPU as a control unit to save processing data, which has to be kept when turning off the power. If the power is not kept during the time, data inside the flash memory, etc. may be destroyed.
To prevent such disadvantages, even when the power is turned off, the power has to be supplied for a while until an operation of turning off the power is completed for performing a saving processing of the memory.
When turning off the power, to supply a voltage desired to be kept until operations of circuits, such as a memory and a CPU, in the electronic device, the countermeasures described below have been taken in the related art.
As shown in FIG. 1, by providing a capacitor 14 having a largish capacitance in a circuit composed of a power supply 11 of, for example, 5V, a constant voltage power supply 12 for outputting a constant voltage of, for example, 2.5V and a circuit unit 13, such as a flash memory, CPU and RAM, so that the capacitor 14 maintains the power supply voltage for a predetermined time required to backup processing even after turning off the power supply 11.
However, in this type of a power supply circuit, the capacitor 14 having a large-size capacitor, that is, a large capacitor has to be provided outside.
Comparing with the circuit configuration shown in FIG. 1, the circuit shown in FIG. 2 is provided with a power supply 15 having a larger current capacity instead of the power supply 11 and provided with a capacitor 14 having a large capacitance at the output side of the power supply 15.
However, in this type of a power supply circuit, the power supply 15 having a large current capacity has to be provided and, furthermore, the capacitor having a large capacitance, that is, a large capacitor has to be provided outside.
A circuit shown in FIG. 3 is provided with a B-system for backup in addition to an A-system composed of a power supply 11 of, for example, 5V, a constant voltage power supply 12 for outputting a constant voltage of, for example, 2.5V shown in FIG. 1. The B-system includes, for example, a second power supply 21 of 5V in a different system from that of the power supply 11, a second constant voltage power supply 22 for outputting a constant voltage of, for example, 2.5V, and a diode 23 for preventing a reverse current flow. Preferably, a Schottky barrier diode having a characteristic of a low voltage drop is used as the reverse flow preventing diode 23.
Normally, the circuit unit 13 such as a flash memory, a CPU and a RAM, etc. is supplied with a voltage of, for example, 2.5V from the first constant voltage power supply 12 in the A-system. A voltage of, for example, 2.5V is also supplied from the second constant voltage power supply 22 in the same way as the first constant voltage power supply 12, but the voltage from the second constant voltage power supply 22 becomes lower than the voltage from the first constant voltage power supply 12 due to a voltage drop (falls) in the reverse flow preventing diode 23, so that power is not supplied from the second constant voltage power supply 22 to the circuit unit 13. Also, a voltage in the A-system is not supplied to the B-system because of the reverse flow preventing diode 23.
When the first power supply 11 is turned off, an output voltage of the first constant voltage power supply 12 is lowered to 0. Since the voltage of the power supply in the B-system is normal, the circuit unit 13 such as a flash memory, CPU and RAM, etc. is supplied with a voltage from a power supply circuit in the B-system, that is, the second power supply 21 and the second constant voltage power supply 22 through the reverse flow preventing diode 23.
Even when using the Schottky barrier diode, showing a low voltage drop as the reverse flow preventing diode 23, however, the forward voltage drops, for example, by 0.4V or so, so that a voltage applied to the circuit unit 13 such as the flash memory, CPU and RAM, etc. becomes 2.1V or so even if a voltage of 2.5V is output from the second constant voltage power supply 22. This voltage is lower by about 20% than the drive voltage of the circuit unit 13 such as the a flush memory, CPU and RAM, etc. operating at a rated voltage of 2.5V, consequently, operations of the circuit unit 13 such as the flash memory, CPU and RAM, etc. may be adversely affected.
To overcome the disadvantages above, the raise of output voltages of the second power supply 21 and the second constant voltage power supply 22 may be considered. However, the reverse flow preventing diode 23 flows a current in a forward direction when a power supply voltage in the B-system is raised, and there is a possibility that power is supplied from the power supply in the B-system to the circuit unit 13 such as the flash memory, CPU and RAM, etc. even in a normal state, so that the power supply circuit may be adversely affected in terms of a current capacity, etc., systematically as the power supply in some cases.
The forward voltage of the diode changes due to a temperature and the characteristics vary due to an element, and thus an unfavorable result has been brought in a low voltage range in some cases.
From the above viewpoint, a memory and a CPU to be used are often operated by relying on a capable performance of being operational even at a low voltage at the time of saving.
Particularly in a flash memory, stored data may become unreadable or destroyed if an operation in the flash memory continues when the CPU normally starts a shut down operation.
FIG. 4 is a view of the configuration of another power supply circuit of the related art.
The power supply circuit shown in FIG. 4 includes an alternating power supply AC of a commercial frequency, for supplying a power to the circuit unit 13 such as a flash memory, CPU and RAM, etc., a power supply switch 32, a first rectifier circuit 10, a DC-DC converter 11, a first constant voltage circuit 12, a standby (backup) transformer 20, a second rectifier circuit 21, a second constant voltage circuit 22 and a reverse flow preventing diode 23.
When comparing the power supply circuit shown in FIG. 4 with that shown in FIG. 3, the power supply switch 32, the first rectifier circuit 10, the DC-DC converter 11 and the first constant voltage circuit 12 are included as a power supply circuit in the A-system; and the circuit illustrated as the first power supply 11 in FIG. 3 is configured by the power supply switch 32, the first rectifier circuit 10 and the DC-DC converter 11 in FIG. 4. Also, the power supply circuit in the B-system is configured by the standby transformer 20, the second rectifier circuit 21, the second constant voltage circuit 22 and the reverse flow preventing diode 23; and the circuit illustrated as the second power supply 21 in FIG. 3 is configured by the standby transformer 20, the second rectifier circuit 21.
In the power supply voltage shown in FIG. 4, in a normal operation, the power supply switch 32 is in a turned-on state, and power is supplied to the circuit unit 13 from the A-system power supply circuit including the first rectifier circuit 10, DC-DC converter 11 and first constant voltage circuit 12. In this state, the B-system power supply circuit, that is, the standby transformer 20, the second rectifier circuit 21 and the second constant voltage circuit 22 are also in operation, but power is not supplied from the power supply circuit in the B-system to the circuit unit 13 due to a voltage drop in the reverse flow preventing diode 23.
For example, when the power supply switch 32 is turned off (an open state), power supply from the A-system circuit to the circuit unit 13 is suspended. However, since the power supply circuit in the B-system is in operation, power is supplied from the B-system power supply circuit to the circuit unit 13 without any discontinuity.
The operation of the power supply circuit in FIG. 4 explained above is the same as that of the power supply circuit explained with reference to FIG. 3.
In the same way as the first constant voltage circuit 12 and the second constant voltage circuit 22 shown in FIG. 3, when assuming that a constant voltage of 2.5V is output from the first constant voltage circuit 12 and the second constant voltage circuit 22 shown in FIG. 4, at the time of supplying power from the B-system power supply circuit to the circuit unit 13, a lack of a voltage to the circuit 13 due to a voltage drop in the reverse flow preventing diode 23 explained above becomes disadvantageous.
On the other hand, when an output voltage of the second constant voltage circuit 22 is raised to improve the voltage shortage, the disadvantageous effects explained above are caused.
The Japanese Unexamined Patent Publication No. 11-175203 discloses a technique of saving data not by using a backup power supply, etc. but by using power generated by using an inertial force of a rotating recording medium.